This invention relates to an improved wedge wire for use in the making of screens for screening suspensions of papermaker""s stock and also to a screen incorporating such a wedge wire.
Apparatus for screening a suspension of papermaker""s stock that use closely spaced bar or wire elements to define screening slots is shown in Martin U.S. Pat. No. 4,267,053 issued May 12, 1981 as an example. The individual wire elements are generally identical to each other in structure are drawn or extruded from metal such as stainless steel and are supported closely spaced relation to each other to define screening slots. The wire elements may extend circumferentially as in the case of a cylindrical screen, or parallel to the screen axis, or any combination of these orientations.
The screening apparatus in which wedge wire screens are used are commonly known as pressure screens in which a stock suspension to be screened or sorted is applied under positive pressure to an inlet of a closed vessel incorporating such a screen. Turbulence-inducing vanes move along the inlet or the outlet screen surface in close relation to the surface and form pulsation waves to prevent the blocking of the screen slots by the material being screened.
Typically a wedge wire is formed with an enlarged upstream end in relation to the direction of flow through the slot formed by the wedge wire. The wire body narrows down at the downstream end so as to present a wedge-like shape in cross section. The individual wedge wires are oriented parallel to or adjacent to each other and are separated by a small defined distance thereby forming screening slots between adjacent wires. The wires are oriented such that the material to be screened enters through slots as defined at the enlarged ends of the wires which together form a screen with a contoured inlet surface.
Typically wedge wires have been formed with flat or planar surfaces joined at relatively sharp corners positioned at the entrance of the screening slot. The direction of the mass flow of the stock being screened is in a direction substantially parallel to the screen inlet surface and therefore at a substantial angle to the flow axis of the slot. The rate of this flow along the screen inlet surface may be at relatively high velocities of about 3,000 feet per minute or more.
The efficiency of the screen in terms of throughput depends on a number of factors but for any given slot width the throughput capacity is substantially limited by the geometry of the slots that are formed by the juxtaposed surfaces of the wires themselves. Abrupt corners presented to the stock suspension at the inlet surface by the wedge wires require the fluid to bend sharply in direction around the wire corners in order to enter the slot. This consumes energy and effectively restricts the rate of flow in such screens.
Further wedge wires commonly do not sharply define and limit the effective length of the slot in the flow direction through the slot and create virtual slots that extend unnecessarily far along the depth of the wires thereby increasing the pressure drop and limiting the flow rate through the screen.
In many wire type screen designs the screening slot is unprotected from direct impact of contaminants in the stock suspension moving in the mass flow rapidly along the inlet surface of the screen. Such impacts cannot only cause premature and rapid wear of the slot, resulting in slot widening, but also can physically damage the slot. It is therefore important that the wedge wire design is one that provides protection for the screening slot. It is also important that the design is one that establishes micro-vortices in the groove area between the slots to assist in the carrying away of rejected particles from the slot back into the main body or flow of the slurry in the pressure screening apparatus for ultimate rejection.
This invention relates to an improved wire element for screening papermaker""s for screening or for sorting a suspension of papermaker""s stock in a bar or wire screen also known as a wedge wire screen and also relates to a screen employing such elements for screening or sorting suspensions of papermaker""s stock. More particularly the invention is directed to improvements in the shape of the wire element such that an upstream surface is formed which hides or protects the screening slot a downstream surface provides for the establishment of micro vortex flow within the groove for carry away rejected particles and at least one of the wall surfaces of the wire element cooperates with an opposing wall surface of an adjacent wire element to define the slot width and the effective slot length measured in the direction of movement through the slot. Further the elements provide an entrance and exit from the slot that is free of sharp corners so as not to impede or restrict the flow of the stock suspension through the screen.
In the improved wire element shape when the wires as assembled in a screen present a profile at the inlet side of the screen in the form of grooves defined by upstream and downstream surfaces of the wires i.e., upstream or downstream of the direction of tangential flow along the inlet surface. These profile grooves have an upstream planar surfaces inclined opposite to the direction of flow by about 20xc2x0 to 40xc2x0 with respect to a reference line which reference line may be a radius line in a cylindrical screen or a median line extending through the slot. The grooves are further defined by downstream surfaces inclined about 40xc2x0 to 80xc2x0 in the direction of flow with respect to such radius or median line.
The slot entry itself is formed by adjacent upstream and downstream surfaces of adjacent or neighboring wire elements. At this region, these surfaces assume an approximately radial orientation or an orientation parallel to a median line through the slot between the surfaces. The transitions from an inclined direction at the inlet surface, to an approximately radial direction are by design, gentle; that is the transitions are formed by relatively large radii on the defining surfaces thereby reducing the pressure necessary to cause flow into the slots between the adjacent screening elements.
At the narrowest cross-section of the slots, at least one of the wall surfaces rapidly diverges from the other to control and minimize the length in the direction of flow of the narrowest portion of the slot to further reduce friction.
More particularly, the wedge wire element shape may be characterized as having an elongated body tapered in the flow direction through the screen with an enlarged upstream end or head in relation to the direction of flow of the stock through the screen and tapering to a smaller end remote from the enlarged end. The body defines over a major portion of its depth generally straight sidewalls leading from the small inner end toward the larger end. The larger end itself is characterized by an upstream plane or surface in relation to the direction of flow of a stock suspension across the surface joining with a downstream surface. The upstream and downstream surfaces join at a common elevated point or region on the head.
The upstream surface is inclined by about between 20xc2x0 and 40xc2x0 to a median line through a slot defined between the element and an adjacent such element while the downstream surface is inclined at an angle of about 40xc2x0 to 80xc2x0 to such median line. In a case of a cylindrically formed screen the median line may also be a radius or may be parallel to a radius.
One of the tapered body sidewalls is joined with the immediately adjacent top wall by a curved protuberance by which the lower end of the upstream wall is joined to a sidewall with the apex of such curved protuberance extending transversely toward such median line and immediately juxtaposed to the junction of the downstream head surface with the sidewall of an adjacent such element thereby forming a slot of a width defined by the spacing of the elements and an effective length in the direction of flow defined by the juxtaposed surfaces. Preferably the protuberance surface forming one wall of the slot is shaped with a relatively constant radius of curvature causing this surface to slope toward a wall of an adjacent such element (defining the narrowest part screening slot at the apex of the curvature) and then sloping abruptly away from the wall to control the effective working depth of the slot. The element on all surfaces is free of sharp protruding edges and all joining surfaces exposed to the inlet flow are radiused. Accordingly the critical slot defined between the elements is characterized by smoothly contoured entrance and exit walls with the effective slot length being controlled by the radius of curvature of the protuberance. Such protuberances may be arranged on either or both of the side walls that define the slot.
The wedge wire elements according to this invention may be provided with integral spacing bumps and mounted to form a pressure screen in the manner described in the PCT application PCT/US99/06120 filed Mar. 26, 1999. The structure and method by which such wires may be formed and mounted as disclosed in the PCT application are preferred although it is within the scope of the invention to mount the elements in known manners as shown for example, in U.S. Pat. No. 5,472,095 or U.S. Pat. No. 5,618,424.
An object of the invention is the provision of an improved bar design for a wedge wire or bar type screen for screening or sorting a suspension of papermaker""s stock characterized by low pressure drop, high efficiency, and long life.
A further object of the invention is the provision of a screen for screening a suspension of papermaker""s stock employing a slotted design using individual closely spaced bar elements as outlined above.
These and other objects and advantages of the invention will be apparent from the following description the accompanying drawings and the appended claims.